Fibre optic networks often employ tunable technologies both for optical add/drop ports and for transmitters. Transmitters using tunable lasers are desirable for several reasons. First, tunable lasers reduce the number of product variants required to construct the network. For example, a Dense Wavelength Division Multiplexed (DWDM) communications band typically has 80 channels. If fixed wavelength (i.e. non-tunable) lasers are used to drive this channel band, then 80 different variants of the transmitter laser are required. This is problematic, especially for customers who must maintain an inventory of appropriate transmitter components for replacement in the case of failure (sometimes called “sparing”). The second reason is that the use of tunable lasers enables re-tuning the transmission wavelength of any given channel in the system for the purpose of reconfiguration, which in turn enables the implementation of a Reconfigurable Optical Add/Drop Multiplexer (ROADM).
The Add portion of a ROADM can be made tunable by including a tunable filter which is tuned in conjunction with the transmitter laser. Prior to the introduction of practical Digital Signal Processor (DSP) based coherent transmitters, it was commonplace to use Distributed Feedback (DFB) or Distributed Bragg Reflector (DBR) tunable laser designs, which have significant out of band noise in the form of side modes and spontaneous emission. This noise needed to be rejected, which drove the need for filtering the laser output light.
Prior to the introduction of practical DSP based coherent receivers, it was commonplace to use direct detection receivers. In a DWDM system, direct detection receivers require optical filters to separate a desired one wavelength channel from the DWDM signal, and present the separated channel light to the receiver for detection. This type of receiver can detect any wavelength which the optical filter chooses. Therefore, the drop portion of the OADM can be made tunable by including a tunable filter.
However, tunable filters are expensive. Reducing the number of tunable filters is advantageous. With coherent transmitters/receivers, it is possible to reduce or eliminate the filtering from the adds/drops. For example, please refer to PCT/CA2009/001455 titled COHERENT AUGMENTED OPTICAL ADD-DROP MULTIPLEXER and filed on Sep. 11, 2009 which is herein incorporated by reference in its entirety. The result is to replace the optical filters with couplers and splitters which are not wavelength selective.
FIG. 1 is a block diagram schematically illustrating elements of an 8-degree Colorless Directionless Contentionless (CDC) ROADM 2 known in the prior art. In the example of FIG. 1, the ROADM 2 generally comprises a respective Wavelength Selective Switch (WSS) module 4 for routing wavelength channels to and from each degree; MUX/DEMUX modules 6 for routing wavelength channels to and from respective transceivers 8; and a Fiber Interconnection Module (FIM) 10 for optically interconnecting the WSS and MCS modules.
Each transceiver 8 typically comprises a transmitter and a receiver (not shown in FIG. 1). In some ROADM systems, the transmitter and receiver are configured to operate at the same wavelength, although this is not essential.
In the illustrated ROADM, each Wavelength Selective Switch (WSS) module 4 is provided as a 1×20 WSS. In the ingress direction, each WSS is capable of routing individual WDM channels received from its respective degree via its common-IN port to any of its twenty output ports. In the egress direction, the WSS can select WDM channels from twenty different input ports and couple them to its respective degree via its common-OUT port.
In the illustrated ROADM, each MUX/DEMUX module 6 is configured using an 8×16 Multi-Case Switch (MCS). In the ingress direction, each MCS 6 is capable of routing optical signals received through any of its eight input ports to any combination of its sixteen transceiver ports. In the egress direction, the MCS couples light received from any combination of its sixteen transceiver ports to any of its eight output ports.
The Fiber Interconnection Module (FIM) 10 is typically provided as a patch panel providing all of the necessary interconnections between the WSSs 4 and MCSs 6. By means of optical connections within the FIM 10, every WSS 4 is connected to every other WSS 4 and to every MCS 6. For example, the FIM 10 provides connections for coupling each of the eight output ports of an MCS 6 to an input port of each of the eight WSSs 4, so that optical signals from all eight degrees can be coupled to any transceiver 8 subtending any one of the twelve MCSs 6. In addition, the FIM 10 provides connections for coupling each of the eight WSS's to all of the other WSS's, so that an optical signal received from one degree can be optically routed to another degree. For the sake of clarity of illustration FIG. 1 only shows optical connections through the FIM 10 between the WSS module 4 on Degree #1 and each of the twelve MCS modules 6 and the respective WSS modules on each of the other degrees. It will be appreciated that each of the optical connections shown in FIG. 1 would normally be implemented as a pair of optical paths to enable bi-directional optical signal flow, and that a corresponding set of connections would be provided for connecting to WSS modules 4 on all of the other degrees both to each other and to each of the MCS modules 6.
Because of the large number of fiber interconnections between the FIM 10 and each of the WSSs 4 and MCSs 6, it is advantageous to make use of multi-fiber patch-cord such as Multiple-Fiber Push-On/Pull-off (MPO) connectors or Multiple-Fiber Push-On (MTP) cables in order to simplify the cabling process. In the example of FIG. 1, each 1×20 WSS 4 includes a conventional duplex Local Connector (LC) 12 to connect its common-IN and common-OUT ports to the fiber pair corresponding to a particular degree, as well as 4 MPO connectors 14, each with 12 optical fibers. Each WSS 4 uses 2×20=40 of the 48 available fibers, leaving 8 unused fibers across the 4 MPO connectors 14. Similarly, each MCS module 6 has 2 MPO terminations 16 (for a total of 24 fibers) carrying 2×8=16 active fibers and 8 unused fibers. Each MCS 6 also has 16 transceiver-facing duplex LCs 18 to connect to up to sixteen individual transceivers 8.
One challenge of using multi-fiber connectors like MPO's is that the user must be able to confirm during the initial installation that all fibers within a particular MPO have continuity and acceptable losses, including fibers that may not be in use initially but that could become active after a node upgrade. The reason for this requirement is that the use of an MPO cable makes it impossible to service individual fibers, i.e. if a problem is found on a previously unused fiber at the time of the node upgrade, all 12 fibers must be disconnected at once to service the cable, which could be interrupting traffic in a section of the ROADM that was already up and running.
It is, therefore, desirable to provide a connection validation technique capable of validating all fibers of a multi-fiber cable, including fibers that are not currently in use.